hydrogels are kinds of crosslinked natural or synthetic polymer materials swollen in water. Because of the good performance in water absorption, water retention and delivery, gels are widely used in industrial, agricultural and medical fields (Qiu, Adv. Drug Deliver. Rev., 2001, 53: 321-339). Relevant products include contact lens, high absorbent resins, plant water retention agents and others.
A regular gel, with a mesh size in the range of several nanometers to tens of nanometers, is generally defined as non-porous. The absorbed water cannot flow freely inside the gel network macroscopically because of the strong interaction. By contrast, gels with pore size ranging from a few microns to hundreds of microns are usually termed as porous gels. The interconnected porous network results to large specific surface areas and high environment sensitivity. These properties benefits for the application in the industry such as wastewater treatment, immobilized enzyme reaction and microbial chromatographic separation (Mattiasson, Trends. Biotechnol., 2003, 21: 445-451).
There are many traditional porous gels preparation methods including phase separation, in situ foaming, using template and the freezing technique (Zhao, Soft Matter, 2008, 4:385-391). Porous gels prepared by phase separation have low porosity and small mesh size. In situ foaming process is difficult to control, so the porous gels have poor mechanical strength. As for using template, the inert template is different to completely remove. The freezing technique shows good performance in preparation of porous gels, because it can easy to regulate pore size, porosity and pore morphology without adding organic solvent or agent. The freezing technique includes cryopolymerization method and lyophilization-hydration method. Cryopolymerization is a method that polymerization of monomeric precursor below the solvent freezing point. During the process, the aqueous phase will freeze and the monomers aggregate together. After the polymerization and warming up, the ice crystals will melt to form porous structure (Lozinsky, Russ. Chem. Rev., 2002, 71:489-511). Conducting several cycles of lyophilization-hydration to a conventional non-porous gel can also introduce porous structure resulting from the destruction of the polymer network (Takahashi, Bull. Chem. Soc. Jpn., 1997, 70:1289-1295). Both the two freezing methods mentioned above have their shortcomings. For the cryopolymerization, it's not suitable for acidic monomers that can't polymerize at low temperature. As for the lyophilization-hydration method, although it can apply to all the conventional monomers, only low porosity and small pore can be obtained because of the elasticity of the gel network. Thus, there is a need for new methods that is able to decouple gel synthesis and porous-forming freezing, to permanently reshape the molecular chain sections opened by ice crystals and, to prevent the polymer network from recovering after the ice crystals melt.